Clark Gable's naked pecs might have started it. In a racy scene with Claudette Colbert in It Happened One Night, Gable unbuttons his shirt to reveal--to the shock of 1934 audiences--a bare chest. No undershirt. Legend has it that undershirt sales dropped 75% that year. While never verified, the tale lives on because Hollywood loves it. If Gable's chest can have that kind of mass cultural impact, the thinking goes, then movies, far from being just passive entertainments, can influence audiences to change their behavior in more significant ways. If a movie can doom undershirts, can't it also end war, poverty,...